[csswg-drafts] Pull Request: [css-color-4] correct gray() example

jonathantneal has just submitted a new pull request for https://github.com/w3c/csswg-drafts:

== [css-color-4] correct gray() example ==
An example of `gray()` in the spec is using a percentage (`50%`), but the syntax for `gray()` should only use a number (`50`):

```
gray() = gray( <number>  [ / <alpha-value> ]? )
```

This is clarified in the description of its arguments:

> The first argument specifies the shade of gray, equal to the **CIE Lightness**...

And it’s further clarified directly below its descriptions:

> In other words, `gray(a / b)` is equal to `lab(a 0 0 / b)`.

See https://github.com/w3c/csswg-drafts/pull/2231